The present invention relates, in general, to an apparatus and method for alloying metals, and more particularly to an apparatus and method for re-flow of multi-layer metal bumps.
Both tape automated bonding (TAB) and flip chip bonding (FCB) of electronic components such as IC chips and passive components often use gold bumps deposited on a device to interconnect them to the next level of assembly, that is the substrate or board. Metallurgical bonds are created during bonding by applying a suitable load and temperature between the gold bumps and corresponding leads (in the case of TAB) or bond pads (in the case of FCB) for times of up to 120 seconds. The total bonding energy as well as the temperature required for this type of assembly has become unacceptably large as the lead count on chips has continued to increase. In addition, semiconductor materials such as gallium arsenide are more fragile than silicon further aggravating the problem.
By replacing the gold-gold bond mentioned above by a gold-tin-gold system the bond loads and times are reduced. This scheme is typically followed by industrial users of the TAB interconnection scheme. By creating a gold-tin eutectic cap on top of the gold bumps prior to TAB bonding, Prof. Reichl's group at the Technical University of Berlin had by 1990 been able to lower the bonding load 3 fold over the unreflowed tin cap method. This was described in the paper "The application of an eutectic gold-tin cushion for TAB inner lead bonding with reduced bonding pressure", Zakel E., Schuler S. and Simon J., Microsystems, Berlin, 1990 and the paper "Au-Sn bonding metallurgy of TAB contacts and its influence on the Kirkendall Effect in the ternary Cu-Au-Sn system", Zakel E. and Reichl H., Proc. 1992 E.C.T.C., 1992, pp. 360-71.
The technique used for reflowing the tin capped wafers described by the Technical University of Berlin group in their papers consisted of heating the wafers with tin capped bumps in a tube furnace at 340.degree. C. with flowing nitrogen for 8 to 10 minutes. The bumps after such processing were found to have several types of defects that adversely affected bonding.
There were two main defects reported after reflow in the tube furnace. The first defect was runoff; that is the tin cap over some of the gold bumps melted and ran down the side of the bumps rather than alloying with the gold to form the desired eutectic. This left behind an inadequate amount of eutectic for bonding. The second defect was termed "mulberries"; rather than forming the desired low melting domed eutectic cap, some bumps after reflow had a granular layer. This second defect was more serious as it made bonding impossible. Often wafers would have mostly "mulberry" type bumps.
For ideal low-load bonding (be it TAB or Flip Chip), it is necessary to create a liquid phase between the bumps and the leads/bond pads. This is possible at a relatively low bonding temperature (280.degree.-310.degree. C.) when a eutectic cap is reflowed over the gold bumps. Zeta phase is an alloy of gold and tin which can form at temperatures as low as 190.degree. C. (lower than the eutectic temperature of 280.degree. C.) yet does not melt below 500.degree. C. The Zeta phase has gold content (&gt;90% by weight) greater than that of the eutectic (80% by weight) and therefore its formation is also favored due to the higher diffusivity of gold into tin compared to tin into gold. If a Zeta phase cap forms over the bump, it cannot be melted below 500.degree. C. a temperature to which neither finished GaAs nor Si devices should be subjected to. Even if this was allowable the resulting bond will not be reliable due to the brittleness of Zeta phase.
There is a need for a method and apparatus for reflow of multi-layer metal bumps which controls run off while simultaneously minimizing both the production of "mulberries" and the production of Zeta phase. The method should be easily controlled and applicable to a wide range of metal bumps.